Directive acoustic pickup



Jan. 7, 1941. I v L R M 2,228,024

DIRECTIVE ACOUSTIC PICKUP Filed Feb. 1, 1940 2 Sheets-Sheet l v INVENTORBY ALEXANDER 'f. ABRAHAMS ATTORNEY Jan. 7, 1941. A, I, AB AH M 2,228,024

DIRECTIVE ACOUSTIC PICKUP Filed Feb. 1, 1940 v 2 Sheets-Sheet 2 INVENTORALEXANDER I ABRAHAM-S BY Patented Jan. 7, 1941 UNITED A STATES PATENTorrics DIRECTIVE ACOUSTIC PICKUP Alexander I. Abrahams, New York, N. Y.

Application February 1, 1940, Serial No. 316,715

'1 Claims. (01. 181-26) This invention'relates to directive acousticpickups and more particularly to the type employing multiple reflectingsurfaces.

An object of this invention is to provide a means for eliminatingacoustic cancellation in deep reflectors due to phase differences at themicrophone, thus producing a deep reflector of higher efficiency.

A feature of this invention resides in the combination of concave andconvex reflecting surfaces :to direct the acoustic rays to themicrophone.

Another feature of this invention resides in the shape of the reflectingsurfaces.

Other objects and features will become apparent from the followingdescription and the accompanying drawings in which like numeralsindicate similar parts and in which:

Fig. 1 is a diagram of the geometrical relationships of doublereflection involved in the present invention,

Fig. 2 is a longitudinal section through the preferred embodiment of myinvention, based on the principles expounded in Fig. 1,

Fig.3 is another embodiment of my invention, wherein the microphone isnot on the primary axis,

Fig. 4 is a diagram of the geometrical relations involved in triplereflections for the purposes of the present invention,

Fig. 5 is a diagram of another type of triple reflector in anotherembodiment of my invention. The construction of the parts and the mannerof assembly will be similar to the preferred embodiment shown in Fig. 2.

point. a

One of the difficulties heretofore encountered in using deep reflectorshas been the large angle with which the acoustic rays converged at thefocal point of the primary reflector.

A microphoneplaced at the said focal point would receive the saidacoustic rays on both faces.

This

would result in acoustic cancellation, at the microphone, of rays whichapproach the said focal point from diametrically opposite directions.

The advantages of this invention reside in the fact that optimalproportions may be selected Through multiple reflection, all of theacoustic rays received by the said concave primary reflector will bedirected to one face of the microphone in a substantially narrow angle.This allows the maximum utilization of the acoustic energy by the 5microphone. i

In Fig. 1, I is a parabolic reflector with axis X'Y and focus at F. 2 isa hyperbola on the same axis XY with one focus at F and a conjugatefocus at M. The dashed line A-B indi-' cates an acoustic ray enteringthe reflector and impinging on the parabolic reflector I at B. The saidray is necessarily reflected by the parabolic reflector I toward itsfocal point F.

Before reaching the focal point F, the reflected I ray impinges on'thehyperbolic reflector 2 at C and is reflected toward the conjugate focalpoint M.

From the definition of a parabola, all rays, entering the parabolaparallel to its axis and reflected to its focus, are equal; or, in Fig.1, AB plus BF is a constant. From the definition of a hyperbola, thedifference between two focal radii drawn from a point on the hyperbolais constant; or, in Fig. 1, CM minus CF is a constant. If we considerthe length of a completely reflected ray from A to M we have AB plus BFplus CM minus CF is always a constant.

In Fig. 2, the concave paraboloid reflector I and the convex hyperboloidreflector 2 are surfaces of revolution about the axis X-Y. Themicrophone 3 is'placed substantially at the conjugate focus of thehyperboloid reflector 2 and is held in place by the spider The entireunit being supported on a standard 5. All rays which enter theparaboloid reflector I parallel to the axis X--Y will be reflectedtoward the microphone 3. The rays will arrive in the same phase sincethe time of traverse will be constant regardless of the position of theentering ray.

The screw and nut 8 at the rear of the concave reflector I which areattached to the convex reflector 2 are used to adjust the position ofthe convex reflector to secure proper focus of the sound waves upon themicrophone.

In the embodiment where the concave and convex reflectors areconstructed integrally, the

screw and nut 8 are omitted.

In Fig. 3, I is a concave paraboloid reflector which is a surface ofrevolution about axis X-Y. 2 is a convex hyperboloid reflector which isa surface of revolution about axis J- K. Axis J--K intersects axis XY atsubstantially the common focal point of the paroboloid and thehyperboloid reflectors. The microphone 3 is substantially at theconjugate focal point of the convex hyperboloid on the axis J-K whichmay be on the periphery of theconvex paraboloid reflector I, or in anyother convenient position. The screw 5 and nut 8 may be used to adjustthe position of the convex reflector. The improvement of the embodimentover that of Fig. 2 resides in the convenient position of the microphoneand in the fact that the said microphone is away from the path of theacoustic rays entering the concave primary reflector.

In Fig. 4, is indicated the geometry of a system employing triplereflection. The acoustic rays enter and impinge on the concaveparaboloid reflector I which reflects the rays toward the focal point F.The convex hyperboloid reflector 2 intercepts and re-reflects said raystoward a second focal'point M. A second convex hyperboloid reflectorintercepts and again reflects the said acoustic rays to a third focalpoint Q which may be at any convenient position.

It will be seen that the number of reflections may be increased withoutlimit, except for the attendant losses of energy due to absorption, andthe expedient position of the microphone. The time relationships of therays arriving at Q will remain unchanged. The total length of thecompletely reflected ray AB plus BF plus CM minus CF plus NQ minus NM isa constant.

In Fig. 5, is indicated the geometry of another system employing triplereflection. The acoustic rays enter and impinge on the concaveparaboloid reflector I which reflect-s the rays toward the 0- cal pointF. The convex hyperboloid reflector 2 intercepts and re-reflects saidrays toward a second focal point M. Intermediate said convex hyperboloidreflector 2 and point M, a plane reflector is placed to re-reflect thesaid acoustic rays to point M.

40 It should be noted that the geometry of Fig. 5 is identical with thatof Fig. 1, except that the plane reflector has rotated the conjugatefocus M to a new position M.

For simplicity of construction, the concave 45 primary reflector l andthe convex secondary reflector 2 may be formed in one operation so thatthe respective surfaces will be the continuation of one another.

I wish it distinctly understood that, although 50 the device has beendescribed as formed by revolving the various conic sections about theirrespective axes, approximate results may be obtained which will differbut little from the exact geometry here developed, by forming thereflect- 55 ing surfaces from their approximate spheroidal or ovatecounterparts. Furthermore, while I have particularly described thesimplest elements adapted to perform the functions set forth, it isobvious that they could be subject to modifica- 60 tions, and variouschanges in form, proportion and in minor details of construction may beresorted to without departing from the spirit or sacrificing any of theprinciples of the invention.

What I claim is:

1. In combination, an acoustic pickup comprising a concave primaryreflector to gather and focus incident sound waves, a secondaryreflecting system positioned within the cavity of the said concavereflector, the said secondary reflect- 70 ing system disposed tointercept and refocus the sound waves substantially to a point forwardof the focal point of the said concave reflector, and

.a microphone positioned substantially at the focal point of the saidsecondary reflecting system.

75 2. In combination with a microphone, an acoustic reflecting systemcomprising a concave primary reflector for gathering and focusingincident sound waves, and a secondary, convex reflector facing towardthe source of the said incident sound waves and positioned within thecavity of the said concave reflector to intercept, re-reflect and focusthe sound waves in a direction substantially toward the source of theincident sound waves; the microphone being positioned at substantiallythe focal point of the system.

-3. In combination with a microphone, an acoustic reflecting systemcomprising a concave primary reflector for gathering and focusingincident sound waves, and a secondary, substantially smaller convexreflector facing toward the source of the said incident sound waves,adjustably supported within the cavity of the said primary reflector andpositioned with one of its foci coinciding with the focus of the primaryreflector and disposed to intercept, re-reflect and focus the said soundwaves in a direction substantially toward the source of the incidentsound waves; the microphone being positioned at substantially the focalpoint of the system.

4. In combination with a microphone, an acoustic reflecting systemcomprising a concave primary reflector of the deep paraboloid type forgathering and focusing incident sound waves, and a secondary,substantially smaller convex reflector of the hyperboloid type withreflecting surface facing toward the source of the said incident soundwaves, positioned within the cavity of said concave reflector with oneof its foci coinciding with the focus of the said concave reflector anddisposed to intercept, re-reflect and focus the said sound waves in adirection substantially toward the source of incident waves; themicrophone being positioned at substantially the conjugate focus of thesaid convex reflector.

5. In combination with a microphone, an acoustic reflecting systemcomprising a concave primary reflector for gathering and focusingincident sound waves, said microphone being positioned at substantiallythe periphery of said primary reflector, a secondary, substantiallysmaller convex reflector facing toward the source of the said incidentsound waves, positioned within the cavity of the said concave reflectorwith one of its foci coinciding with the focus of the said concavereflector and disposed to intercept, rereflect and focus the said soundwaves in a direction susbtantially toward the source of the incidentwaves, a third reflector disposed to intercept the sound waves from thesaid secondary reflector and to re-direct them to the microphone.

6. In combination with a microphone, an acoustic reflecting systemcomprising a concave primary reflector for gathering and focusing.incident sound waves, and a secondary, substantially smaller convexreflector facing toward the source of the said incident sound waves andpositioned within the cavity of said concave reflector, to intercept,re-reflect and focus the sound waves in a direction substantially towardthe source of the incident sound waves, the microphone being positionedat substantially the focal point of the said convex reflector; the saidprimary and secondary reflectors being of integral construction.

7. In combination with a microphone, an acoustic reflecting systemcomprising a concave primary reflector for gathering and focusingincident sound waves, a secondary, substantially smaller convexreflector facing substantially toward the source of the said incidentsound waves and positioned within the cavity of said concave reflectorto intercept, re-reflect and focus the sound waves in a directionsubstantially toward the source of the incident sound waves, one of thefoci of the said secondary reflector coinciding with the focus of thesaid primary reflector, the

axes of the two reflectors being inclined to each conjugate focus of thesaid secondary reflector. 5

ALEXANDER I. ABRAHAMS.

